The present invention generally relates to rugate filters and, in particular, relates to one such rugate filter having suppressed harmonics.
Conventional optical filters are usually fabricated by applying a plurality of discrete layers of material to an optical substrate. The material of each of the layers is selected so that there is a change in the index of refraction at every interface. More specifically, the material of the layers is chosen so that the index of refraction alternates from a comparatively higher (or lower) value to a comparatively lower (or higher) value at each layer interface.
As known in the field of optics, when light impinges upon any interface where there is a difference in the index of refraction some of the incident light will be reflected. Further, at an interface whereat the incident light traverses from a material of relatively high index of refraction to a material of relatively low index of refraction a phase change of 180 degrees is introduced into the reflected light. Hence, there is, with the appropriate choice of layer thickness, destructive cancellation of the reflected light at consecutive interfaces. Consequently, the more interfaces an incident light beam traverses, the greater the amount of the incident light reflected and canceled. When substantially all of the light of a particular wavelength is reflected and canceled before reaching the optical substrate, that wavelength of light can be said to have been filtered, or rejected, from the incident light beam. It is known and understood that, with such stacked arrangements, not only is the principle wavelength rejected but the harmonics thereof are also rejected.
More recently, rugate filters have been developed. In the case of rugates, rather than forming a plurality of discrete layers of material onto an optical substrate, a single layer of material is formed in such a fashion that the index of refraction varies within the layer itself. Typically, such rugates are effected by a continuous deposition process during which one or more of the materials deposited are varied. Hence, the variation of the index of refraction.
In one particular application, i.e., single wavelength rejection filters, the typical rugate will have an index of refraction versus optical thickness profile through the layer that is sinusoidal. As used herein the term "optical thickness" is taken to mean the product of the mechanical thickness and the index of refraction. However, other profiles can be implemented depending upon the optical characteristics desired. In general, multiple reflection bands can be generated by superimposing the individual refractive index profiles and depositing the resulting profile. For example, if a number of wavelengths are to be suppressed, the profile of the final rugate would be the resultant profile of the sum of the individual profiles desired.
One of the advantages of rugates is that the resultant layer is quite thin. In fact, the typical rugate layer is sufficiently thin that incident light ray deviations due to changes in the index of refraction is negligible.
While a sinusoidal rugate exhibits some harmonic suppression compared to comparable stacked single wavelength rejection filters, the harmonic content remains significant. In fact, it has been found that the conventional superimposing of the individual refractive index profiles does not seem to apply with respect to the rejection of the harmonics of the principle wavelength.
The rejection of harmonics is an important consideration regardless of the intended use of the rugate filter, however, it becomes extremely important when the rugate is used to reject laser light of known wavelengths. The rejection of laser light is important for both domestic and military applications.
The problem of suppressing harmonics in rugates has been given much consideration in the art. For example, one approach has been to superimpose an elliptical function with the principle sinusoidal filter function. Such an approach is discussed in an article by W. H. Southwell, entitled "Rugate Index Profile Which Suppresses All Harmonic Stopbands", 1988 Technical Digest Series, Vol 6. However, this approach does not correct for what seem to be manufacturing anomalies that appear to be inherent in many rugate manufacturing processes. For example, it is not unusual to find strong harmonics even when the various corrective techniques have been implemented. Consequently, a rugate filter having suppressed harmonics is clearly desirable and needed.